X-ray imaging techniques based on Compton backscatter allow inspection and screening of cargo sea- and air-containers, a variety of vehicles, small ships, luggage, and suspicious packages. Government agencies, border authorities, law enforcement personnel, military organizations, and security services in more than 65 countries widely use such systems. For example, more than 750 mobile backscatter systems sold to date. In contrast to commonly used transmission inspection systems, backscatter imaging involves positioning both source and detectors on only one side of a target object. Such systems are exclusively useful in situations where access to the far part of the inspected object is limited, making X-ray transmission system impractical.
Compton backscatter imaging (CBI) is a single-sided imaging technique in which the radiation source and the detection/imaging device are located on the same side of the object. As a result, CBI is a valuable non-destructive inspection (NDI) tool because of its single-sided nature, the penetrating abilities of radiation, and unique interaction properties of radiation with matter. Changes in the backscatter photon field intensity (resulting in contrast changes in images) are caused by differences in absorption and scattering cross sections along the path of the scattered photons. Since the inception of CBI, a diverse set of imaging techniques have evolved using both collimated and un-collimated detectors, coded apertures, and hard X-ray optics. “Pencil beam” CBI uses a highly collimated beam of radiation to interrogate objects. The pencil beams may vary in diameter from microns to centimeters, but usually consist of a near-parallel array of photons forming a tight beam. A detector measures the backscatter from the CBI pencil beam as it scans the object.
The rate at which the X-rays are scattered from a region is indicative of its density; since Compton-scattering of X-rays takes place from electrons of individual atoms, less scattered X-rays emerge from the material when the material is denser. This fact is known to the prior art to be useful in generating images of structures using Compton scattering.
The attenuation of photons of the energies less than 1 MeV is composed of Compton scattering, photoelectric absorption and to a small degree coherent scattering. FIG. 3 shows the contribution of Compton scattering to the total attenuation for several elements as a function of energy. In the region up to 1 MeV, the dominant process of interaction is photoelectric absorption, which is raises with atomic number. Therefore, for light elements backscatter radiation will be more intensive compare with materials with higher Z. This is the reason why in backscatter images light object (low Z) looks brighter compare with high Z objects.
The backscatter signal is dependent upon the atomic number, density and thickness of the material under test and the incident energy spectrum of the photons. FIG. 4 shows the total backscatter as a function of thickness of acrylic and aluminum for two Bremsstrahlung incident spectra at 55 and 110 keV.
At 110 keV an infinite thick acrylic layer would backscatter approximately 25% of the incident energy and an aluminum layer about 7%. The depths where 75% of the maximum backscatter is reached are 5 cm in acrylic and 1 cm in aluminum.
Conventional backscatter inspection systems have a significant limitation in their ability to penetrate even moderately dense cargos. Moreover, the signal is dominated by the first interrogated layer, typically metal wall of the container or a vehicle or the fiberglass hull of a boat.
FIG. 1 illustrates a conventional X-ray backscatter cargo inspection system as has been described in D-C. Dinca, J. R. Schubert, J. Callerame. X-ray Backscatter Imaging, Proc. Of SPIE, Vol. 6945, 694516, (2008) doi:10.1117/12.773334, which is incorporated herein by reference.
The X-ray pencil beam is created by the rotating collimator 107 on the right and is scanned vertically while the object being inspected moves horizontally. The X-ray beam 102 itself passes between two large backscatter detectors 104, and scattered X-rays 103 are collected and registered by detectors 104 at each beam position.
To overcome this fundamental limitation, we have developed an advanced concept for backscatter system that uses the intrapulse ramp of the electron beam energy of the X-ray source. We propose improving on the conventional backscatter systems based on the continuous beam X-ray tubes, or “quasi-continuous” linac with energy- and current-modulated pulses, fast X-ray backscatter detectors, and algorithm of image “peeling” processing.
In this approach, we temporally encode the sensitivity to multiple layers of the imaged cargo. By sequencing the end-point energy of the X-ray beam in a predictable manner, we separate the backscatter signals originating from various depths of the cargo. The proposed concept represents an extension of the approach proposed for transmission inspection systems as have been described in A. Arodzero. Scintillation-Cherenkov detector and method for high-energy X-ray cargo container imaging and industrial radiography, US Patent Application 2011/0163236 and A. Arodzero, S. Boucher, A. Murokh, S. Vinogradov, S. Kutsaev, System and Method for Adaptive X-ray Cargo Inspection, US Patent Application 2015/0338545 and A. Arodzero, S. Boucher, J. Hartzell, S. Kutsaev, R. C. Lanza, V. Palermo, S. Vinogradov, V. Ziskin, High Speed, Low Dose, intelligent X-ray Cargo Inspection. IEEE-2015 Nuclear Science Symposium proceedings, paper N2B1-6 which are incorporated herein by reference.